When a subsistence farmer in Asia wishes to divide the irrigation water into two or more parts, he simply hoes out a shallow trough in the sides of the main irrigation ditch to conduct water to lateral irrigation ditches. If two of the shallow troughs are near each other and about the same shape, length and depth (as measured from the water surface), the flow will be divided more or less evenly. This sort of liquid flow dividing is what the proposed invention is for, but the proposed invention has unique features that make it vastly superior to these hoed troughs. Both my invention and the hoed troughs are simply types of weirs, however and thus are variants on a very old concept.
One use for this invention is when the head at the two weirs is slightly different, and yet we want approximately equal flow from each. This difference might be due to vertical misalignment of the weirs, surging due to flow patterns, or other causes. In such a case division of the flow by a V-notch weir or a rectangular notch weir is generally unsatisfactory due to the large difference in the flow over the two weirs. This is especially true at low flows where there may be little or no flow over the weir with the lower head. A normal "V" notch weir will almost triple the flow if the head above the notch bottom changes by a factor of 1.5. This implies that even a slight vertical misplacement of one of the weirs would result in a considerable difference in the amount of flow through that weir. This is one of the central problems in using weirs to divide the flow.
Prior art efforts to solve these problems have included narrow rectangular notch weirs. This does minimize the problem at low flows because the total head is raised making the head differences small compared to the total head. There are at least two significant problems with this approach. First, the narrow opening is quite subject to clogging in many (if not most) open channel applications. Second, it has a very small dynamic range. That is, if it is accurate in dividing the flow at, say, 0.2 gallons per minute, the maximum flow for a device with reasonable total height might be only 0.5 gallons per minute.
In order to overcome these disadvantages prior art includes the modified combination V-notch and narrow rectangular notch. In the modified combination V-notch, the V-notch has a narrow rectangular notch extending downward from the bottom of the "V". This device does solve the problem in part, but also has some disadvantages. The low flow portion (the narrow slot) is still subject to clogging. The high flow region uses the inaccurate V-notch so that a small error in vertical alignment or head will cause a significant error in flow dividing for medium flows. If there is a vertical misalignment, there would be no way to know what the percentage error between outputs would be and that percentage error would be a changing function of the head. The present invention greatly reduces these problems, and also provides a set of weirs with a known minimum accuracy for a given maximum vertical positioning error among the weirs.
Another important use for these weirs is to divide a flow into any specified ratio by the use of such weirs. This invention allows this to be accomplished so that this ratio will be maintained over a wide range of flow with no adjustment in the weirs. There is little prior art along these lines, though the need for such a device is obvious.